Method to manufacture pellet of nuclear ceramic fuel. RU patent 2504029.

FIELD: power engineering.

SUBSTANCE: pellet of nuclear fuel from uranium dioxide with homogeneously distributed oxides of aluminium and silicon and required content of aluminium from 0.005 to 0.03 wt % and silicon from 0.003 to 0.02 wt % is manufactured by introduction of a master powder of uranium protoxide-oxide U₃O₈ at the stage of preparation of the pressed powder in the amount of up to 30 wt %. At the same time the master powder is prepared in accordance with the ADU-process from a solution of uranyl nitrate containing aluminium and silicon in quantities from 0.05-0.3 wt %.

EFFECT: increased depth of fuel burning.

1 dwg

The invention relates to nuclear technology, in particular, to the technology of oxide nuclear fuel for fuel elements of nuclear power plants.

To ensure availability of fuel at up to 70 MW day/kgU and above, you must meet certain requirements on the microstructure of sintered pills to exclude the increased output of gaseous fission products (GPA)and high swelling of the fuel.

According to calculations, the average size of a grain microstructure of sintered tablet fuel intended for 5...7-year-long campaign must be not less than 50-60 microns.

At the same time, we must note that the amount of the average grain size of a microstructure of sintered tablets achieved to date production is less than 20 microns:

- from 11 to 14 microns up to 16-18, during the processing of uranium dioxide powders, obtained by the ADU-process.

- 10 to 14 microns using powders, obtained by «dry conversion», and

- 8-10 microns during sintering of pellets from uranium dioxide powders, obtained by gas-flame technologies.

One of the main directions of improvement of characteristics of fuel and optimization of its microstructure becomes supplementation oxides to the source of uranium dioxide powder (see J.K. Б, A.V. Medvedev, O. Milovanov. The relationship of the initial parameters of fuel pellets with their properties and with the basic performance characteristics of the fuel rod.

Development of perspective requirements to the fuel tablets. VNIINM. M., 1997.) For obtaining sintered tablets with a coarse-grained microstructure is proposed to introduce into the source of uranium dioxide powder additive in the amount of thousandths to one hundredth of percent (alloying oxides of aluminum, silicon, niobium, vanadium, chromium, titanium, and other elements). An important problem with this is to provide a homogeneous distribution of the alloying additive.

There is a method of increasing the size of a grain of sintered pills by mixing uranium dioxide UO 2 with the addition of, containing in its composition phase. The grain size of the tablets is 20 to 60 microns (see EPO patent, IPC G21C 3/62, 1 0502395, 1992). phase containing 40 to 80% SiO 2 , (the rest are aluminum oxide (Al2O3 ), pre-prepared baking powders of aluminum oxide with silicon oxide and then crushed sintered aluminosilicate.

There is a method is characterized in that in addition to aluminium oxide additive is one of oxides number: TiO 2 , Nb 2 O 5 , SiO 2 , CaO, MgO, BeO, Na 2 O, P 2 O 5 in an amount to provide content in a tablet aluminium and element from the specified a number within 20-500 ppm each element in relation to uranium, with a ratio of 60-85% aluminum, the rest element from the specified number of which is entered without prior its sintering and the subsequent grinding directly in the process of preparation (see RF patent №2268507 IPC G21C 3/62, G21C 21/00, 2005).

In another known method (patent of Russian Federation №2339094 IPC G21C 3/62, 2008), which is proposed for the fabrication of fuel with the addition of erbium oxide, at the stage of preparation entered activating the growth of grain additives of aluminum oxide powder with a powder in one of oxides number: TiO 2 , Nb 2 O 5 , SiO 2 , CaO, MgO or enter only one mineral additive aluminosilicate (zeolite, metakaolin, , montmorillonite, vermiculite).

To improve the homogeneity of the distribution of alloying additives, proposed a method of manufacturing of the fuel pellet, in accordance with which the powders of aluminum oxide and silicon crushed to a particle size less than 40 microns and injected into the plasticizer, for example, in aqueous solution of polyvinyl alcohol and glycerine, which is used in preparing uranium dioxide. Before chopping aluminium oxide powder pre-annealed in air at a temperature of 700 to 800 degrees C (see RF patent №2376665 IPC G21C 3/62, 2009).

The important point of the development of the technology of manufacture of tablets from micro additives oxides of alloying is to provide high quality mixing of powders. The technical difficulties of this operation are due to the very small quantity of the entered additive.

There is a method of introduction of the alloying additive in the form of oxide aluminum, calcium, magnesium, titanium, zirconium, vanadium, niobium, or a mixture thereof at the stage of the formation of ammonium diuranate. ammonium mixed with 0.05 to 1,70 mol.% (no. UO 2 ) additives, containing at least one of the above alloying elements. The mixture is then calcined and reduced to UO 2 . In addition, the introduction of alloying can be done at the stage of hydrolysis in the production of UO 2 by way of a dry conversion (see EPO patent, IPC G21C 3/62, №0076680, 1984). The disadvantage of this method is the introduction of the press-powder coarse-grained aluminum oxide and silicon.

This method is the closest to the technical nature of the claimed method and is taken as a prototype.

Object of the present invention is to increase the degree of homogeneity of the distribution of alloying additives in preparing , improving the reliability of education homogeneous mixture in the case of the presence in the mixture components, the content of which is at the level of 20-300 ppm.

This problem is solved using the proposed method of manufacture tablets nuclear ceramic fuel.

Summary of the invention consists in the fact that unlike the known method, consisting in the introduction alloying additive in the form of oxide aluminum, calcium, magnesium, titanium, zirconium, vanadium, niobium, or a mixture thereof at the stage of the formation of ammonium diuranate, subsequent annealing ammonium diuranate to nitrous oxide and recovery of uranium to uranium dioxide, the proposed method of mixing of components of conduct on the ionic level in the solution of uranyl nitrate, which introduced the estimated number of soluble compounds of aluminum and silicon, in 10-30 times higher than the nominal content of such impurities in tablet.

During the subsequent interaction of this solution with ammonia water along with the deposition of ammonium diuranate is happening co-deposition of aluminium and silicon. As a result of annealing ammonium diuranate air produces nitrous oxide of uranium, which homogeneously distributed oxides of aluminum and silicon, the content of which is 10-30 times higher than the nominal content of such impurities in tablet.

Powder, uranium oxide concentrate, with impurities of aluminium and silica resulting from the calcination of ammonium diuranate, is then used as the master powder for the preparation of uranium dioxide.

Adding powder oxide to uranium dioxide in preparing - common technique in the production of ceramic fuel, which allows not only to increase the degree of utilization of raw materials, but also to some extent improve the characteristics of sintered tablets. When used in such cases, powder oxide uranium with a complete specific surface area of about 1 m 2 /g, prepared by oxidation of non-conforming sintered tablets, the number of additives of nitrous oxide in not exceed 10-15%by weight.

In contrast powder, obtained by the proposed method wizard powder, uranium oxide concentrate has a highly developed specific surface and in its particle size distribution are nano-sized fractions with particle size of less than 100 nm (less than 0.1 micron). Previous studies have shown that in the press-powder, you can add up to 30 wt.% such a nitrous-oxide, without fear of worsening the quality of sintered pellets.

It should also be noted that the restoration of the nanostructured powder oxide in the heating process tablets in a reducing atmosphere, sintering furnaces, formed a nanostructured uranium dioxide additive which promotes the growth of grain microstructure pills (patent RF №2186431 IPC G21C 21/00, G21C 3/02, C01G 43/025, 2002)

Example of the method

Master powder oxide, mixed with aluminum and silicon received a well-known method for ADU-method:

to 1500 ml of a solution of uranyl nitrate (with a uranium content of 73.8 g/l) was added with stirring 200 ml aluminum (aluminum content 5.5 g/l) and 150 ml of a silicon solution (containing silicon 2.5 g/l). Cooked mixed solution was poured simultaneously with the 25%ammonia water in the buffer and conducted deposition of ammonium diuranate at a value of pH 6.8 at the first stage and pH=8,9 on the second. The resulting salt dried at a temperature of 110-120°C, rubbed through a sieve with 0,2 mm and at a temperature of 600 degrees C in air. So prepared master powder, uranium oxide concentrate (with oxides of aluminum and silicon) had the full value of the specific surface 11.4 m 2 /g and contains about 1.5% fraction of particles with size less than 100 nm.

For the preparation of used factory uranium dioxide powder, obtained by the method of rehabilitation of uranium hexafluoride. The full the specific surface of the powder 2.8 m 2 /year

prepared by adding to uranium dioxide powder 5 and 10 wt.% master powder uranium and 0.3% of the dry lubrication. After thorough mixing of the mixture extruded at a specific pressure 2100-2200 kgf/cm 2 in a matrix in diameter 9,3 mm Pressed tablets at a temperature of 1750 C; holding at that temperature was 4 hours. Figure 1 shows the microstructure of sintered pellets and distribution of grain size, determined using optical microscopy.

In both cases, the average size of a grain microstructure of sintered pills exceeds 30 microns.

A method of manufacturing pills nuclear ceramic fuel on the basis of uranium dioxide with homogeneously distributed oxides of aluminum and silicon aluminum content of 0,005 to 0,03% Mas. and silicon from 0.003 to 0.02% including the preparation of with the addition of aluminum oxide and silicon, characterized by the fact that at the stage of preparation of the press-powder to uranium dioxide powder add up to 30 wt.% master powder uranium containing 0.05 to 0.3 wt.% aluminium and 0.03-0.2 wt.% silicon and prepared by annealing at a temperature of 600-650°C ammonium diuranate, which is besieged by the ADU-method ammonia water solution of uranyl nitrate, containing aluminium and silicon.